An ignition coil for an internal combustion engine has a coil body part generating high voltage and a joint part, which holds therein a conducting member electrically connecting the coil body part and a spark plug. The joint part of the spark plug is inserted in a plug hole of an engine head. Then, the conducting member of the ignition coil is electrically connected with the spark plug at a terminal fitting. In addition, the spark plug is attached to the engine head at a housing.
Here, a joint part of the ignition coil has a cylindrical plug cap into which an insulator of the spark plug is fitted. Patent literature 1 discloses a structure in which a plug cap is formed so that the inner peripheral surface thereof has a convexo-concave shape in the axial direction to attach a spark plug to the plug cap and detach the spark plug from the plug cap.